while starting firefox u have to create two separate profiles of firefox so that u can login to two accounts of any(orkut,yahoo,rediff or anything else).......
for three logins create three profiles of firefox.........

Trick 2

login in firefox?
CookieSwap enables you to easily swap all your cookies so that you can be logged in to multiple web e-mail accounts (like Gmail and Yahoo! mail) at the same time and quickly switch between them.